The p53 tumor suppressor gene is inactivated by missense mutations or deletion in over half of all human cancers. Biochemically, p53 functions as a sequence-specific transcriptional activator to induce genes involved in DNA repair, cell cycle arrest and apoptosis in response to genotoxic stress. These responses are essential to prevent the emergence of cells with unstable genomes, which are prone to oncogenic transformation. Thus, transactivation of specific genes is critical for p53 function. Coactivator complexes which link transcriptional activators, bound to specific promoters, to histone acetylation and basal transcriptional machinery, are essential for the function by sequence-specific transcription factors. We have identified the human ADA3 (alteration/deficiency in activation) protein as a novel p53-binding partner. In the yeast, ADAS is an essential component of the ADA coactivator complex that include ADA2 and GCN5 (general control non-repressed 5), a histone acetyl transferase (HAT). Only recent studies by us and others have begun to characterize the mammalian ADA complexes. We have demonstrated that ADAS directly interacts with p53 and enhances its transactivation function by promoting its acetylation and stability. shRNA-mediated knockdown of ADAS indicates that it is required for p53 acetylation and stabilization upon DNA damage. Mutant p53 that can not be acetylated on major p3OO acetylation sites is not stabilized by ADAS. Collectively, these findings lead us to hypothesize that hADAS is a central coactivator component that recruits p3OO/CBP and other HATs to acetylate and stabilize p53 on its target gene promoters leading to enhancement of p53-mediated function. Together, ADA3-dependent histone and p53 acetylation provide critical mechanisms for p53-mediated DNA damage response. To test these hypotheses, we will examine the role of ADAS in the recruitment of major HAT proteins (p3OO/CBP, hGCNS and PCAF) to p53. Furthermore, we will define the relative role of ADAS in p53-mediated cellular responses upon DNA damage (UV. radiation and chemicals) using cell culture models and ADAS conditional knockout mice (for UV and carcinogen-induced tumorigenesis) that we have generated. These analyses are likely to define a novel biochemical pathway to regulate p53-mediated cellular responses following genotoxic stress, and the relevance of the pathway as a barrier to oncogenic transformation. Elucidation of the role of this new biochemical pathway and definition of its components is likely to provide new targets for future development of gene- and protein-based diagnostic and therapeutic strategies for human cancer.